1. Field of Invention
This invention relates to integrated circuit analog amplifiers and more specifically calibrating the frequency response of operational amplifiers.
2. Description of Related Art
Operational amplifiers are found in a wide array of usages, and many of these operational amplifiers are integrated into a semiconductor chip. The ability to measure and compensate the integrated operational amplifiers are somewhat limited without using chip signal I/O and chip real estate. Process variations along with variations in temperature caused by differences in the environment in which the chips are placed can cause changes in the margin of stability of the operational amplifier. These factors along with the aging of the component parts of the operational amplifier can produce a situation which is detrimental to the stability of the amplifier by producing less phase margin or be detrimental to the operation of the amplifier by producing too much phase margin.
In U.S. Pat. No. 5,642,078 (Navabi et al.) an amplifier is disclosed that has frequency compensation using gain degeneration. The amplifier is compensated by dynamically varying the transconductance of a stage according with the gain of the output stage of the amplifier. In U.S. Pat. No. 5,117,200 (Scott, III) a wide bandwidth transconductance amplifier is stabilized over a wide range of output currents by using a compensation driver circuit to sense the output current and feed the information back through a compensation capacitor.
The stability of an amplifier can be measured by the gain margin or the phase margin of the amplifier. Where gain margin is the open loop gain required to make the amplifier unstable, and phase margin is the open loop phase shift required to make the closed loop amplifier unstable. Both these measurements of stability measurements would be difficult and laborious to make on an integrated circuit, and once made corrections would be as difficult. A way is needed to automatically make measurements on the integrated amplifier and correct for shifts in the characteristics of the amplifier that can be caused by differences in manuafacturing, changes in the environment such as temperature and aging.